When you buy a gallon of organic milk, you expect to get tasty milk from happy cows who haven't been subjected to antibiotics, hormones or pesticides. But you might also unknowingly be getting genetically modified cattle feed.

Albert Straus, owner of the Straus Family Creamery in the small northern California town of Marshall, decided to test the feed that he gives his 1,600 cows last year and was alarmed to find that nearly 6% of the organic corn feed he received from suppliers was "contaminated" by genetically modified (GM) organisms. Organic food is, by definition, supposed to be free of genetically modified material, and organic crops are required to be isolated from other crops. But as GM crops become more prevalent, there is little that an organic farmer can do to prevent a speck of GM pollen or a stray GM seed from being blown by the wind onto his land or farm equipment and, eventually, into his products. In 2006, GM crops accounted for 61% of all the corn planted in the U.S. and 89% of all the soybeans. "I feared that there weren't enough safeguards," Straus says.

So Straus and five other natural food producers, including industry leader Whole Foods, announced last week that they would seek a new certification for their products, "non-GMO verified," in the hopes that it will become a voluntary industry standard for GM-free goods. A non-profit group called the Non-GMO Project runs the program, and the testing is conducted by an outside lab called Genetic ID. In a few weeks, Straus expects to become the first food manufacturer in the country to carry the label in addition to his "organic" one. With Whole Foods in the ring, the rest of the industry will soon be under competitive pressure to follow.

Earning the non-GMO label, at least initially, requires nearly as much effort as getting certified organic. To root out the genetically modified corn, Straus spent several months and about $10,000 testing, re-testing and tracing back his products: from his own dairy's milk, to other dairies that supply some of his milk, to the brokers who sell them feed, to their mills that grind the corn, to farmers who grow it. To put the GM-free label on his ice cream, Straus will have to trace the chickens that provided the egg yolks, the grain used in the alcohol that carries his vanilla extract and the soy lecithin used as an emulsifier for his chocolate chips.

So why bother? The organic and natural foods industry sees a huge opportunity in telling consumers even more about what's in their food. Few consumers would think about the pesticides and hormones in conventional foods without the organic alternative to remind them. Similarly, genetically modified crops have become so prevalent in the U.S. that chances are you've been buying and eating them for years. You just wouldn't know it from the label: the U.S. Department of Agriculture, unlike agencies in Europe and Japan, do not require GM foods to be labeled. While scientists have not identified any specific health risks from eating GM foods, anti-GM activists say there is not enough research yet into their long-term risks or impact on biodiversity. By telling consumers loud and clear which products are GM-free, organic-food producers will give them one more reason to choose organic. Says Jeffrey Smith, a longtime activist against genetically modified food: "The people served by the organic industry are very sensitive to GMO." And, the industry hopes, willing to pay to avoid it.